Optimized implementation of SkMatrix44::preScale
Optimized implementation of SkMatrix44::preScale

preScale() does not need to do a general matrix multiply.  It can do 4x less
arithmetic by directly scaling the columns of the input matrix.
Review URL: https://codereview.appspot.com/6923048

git-svn-id: e8541e15acce502a64c929015570ad1648e548cd@6807 2bbb7eff-a529-9590-31e7-b0007b416f81

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